SI base unit
The (SI) defines seven units of measure as a basic set from which all other SI units are . These SI base units and their physical quantities are: * metre for (US English: meter) * kilogram for mass (note: not the ) * second for * for * kelvin for * for * for the . The SI base quantities form a set of mutually independent dimensions as required by commonly employed in science and technology. However, in a given realization of these units they may well be interdependent, i.e. defined in terms of each other. The names of SI units are written in lowercase characters (the 'degree Celsius' meets this rule, as 'degree' is the unit, and 'Celsius' is a modifier). The symbols of units are written in lowercase (e.g. meter has the symbol m), except that symbols for units named after persons are written with an initial capital letter (e.g., the hertz has the uppercase symbol Hz). Many other units, such as the , are formally not part of the SI, but are . __TOC__ Future redefinitions There have been several modifications to the definitions of the base units, and additions of base units, since the in 1875. Since the redefinition of the meter in 1960, the kilogram is the only unit which is directly defined in terms of a physical artifact rather than a property of nature. However, the mole, the ampere and the candela are also linked through their definitions to the mass of this – cylinder stored in a vault near Paris. It has long been an objective of to find a way to define the kilogram in terms of a , in the same way that the meter is now defined in terms of the . The 21st (CGPM, 1999) placed these efforts on an official footing, and recommended "that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." Two main possibilities have attracted attention: the and the . In 2005, the (CIPM) approved the preparation of new definitions for the kilogram, the ampere, and the kelvin and it noted the possibility of a new definition for the mole based on the Avogadro constant.94th Meeting of the (2005). Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants The 23rd CGPM (2007) decided to postpone any formal change until the next General Conference in 2011.23rd (2007). Resolution 12: On the possible redefinition of certain base units of the International System of Units (SI). In a note to the CIPM in October 2009, Ian Mills, the President of the CIPM Consultative Committee - Units (CCU) cataloged the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposed . He urged the CIPM to accept the proposed changes in the definition of the kilogram, ampere, kelvin and mole so that they are referenced to the values of the fundamental constants, namely (h), the (e), (k), and (NA). See also * * * * es * * * * * * References External links *BIPM *National Physical Laboratory *NIST -SI Base unit Category:SI base units Category:Dimensional analysis